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Yamaha C-4 Preamp Recap

I'm thinking D311 is possibly bad . Remove TR634..and re-assess the previous voltage measurements and confirm my suspicion . :biggrin:
 
FYI -- D311 is and VD1212 varistor ( small black bead with a red spot) . Typically a pair of 1N4148 diodes are wired in series and substituted.
 
Jake O Bob said:
Do I reinstall TR632 before I remove TR634? Or have them both removed?
Click to expand...
No..TR632,633 and 634 are crowbar transistors .Designed to shut down the power supply if a fault occurs.
 
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Test TR628 out of circuit..It is possibly shorted. Or replace it with a good new part.
 
Tested TR628 out of circuit. Pretty sure it's shorted. Thanks for getting me this far.

When finding a replacement on mouser, what am I looking for? Service manual in one place says 2SA941 and the other says 2SA777 :confused:. Both say 1E, 2C, 3B so maybe it doesn't matter too much? Am I looking at the ECB voltages in the SM to decide what one matches closest on mouser?

Should I pickup some 1N4148 diodes as well (as noted above)? Or forget about those?
 

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Replaced TR628 with new transistor. Also replaced D311 with 2x 1N4148 diodes in series. Reinstalled TR632 and TR634. Same result as before, power lights flash on for a second, then go out.

I removed TR632 from circuit again to get more measurements, but not sure what to diagnose or what to look for now.
 
Jake O Bob said:
Reinstalled TR632 and TR634
Click to expand...
Leave TR632,633 & 634 out of circuit until further notice. If there is and issue with the power supply . These three transistors will shut down the power supply.( ie. crowbar )
I can't T-shoot the power supply with it being shut down.
 
Jake O Bob said:
If TR628 is 2SA777 (based on service manual) then measurements are

E -.93v
C -43.2v
B -1.2v
Click to expand...
Remove D308 and D312 -- test them out of circuit. Better yet -- swap them out with a couple 1N4148's.
 
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